Inactivation of E. coli O157:H7, Salmonella enterica, and L. monocytogenes through semi-continuous superheated steam treatment with additional effects of enhancing initial germination rate and salinity tolerance

Abstract

Superheated steam (SHS) is a powerful technology used to reduce bacteria on food surfaces while causing less damage to the underlying sublayer of food compared to conventional heating treatments. In this study, a semi-continuous SHS system was developed to inactivate foodborne pathogens within 1 s (Escherichia coli O157:H7, Salmonella enterica, and Listeria monocytogenes) on radish seed surfaces and to enhance the seeds’ salinity tolerance, which is vital for adapting to arid and semi-arid regions. The temperature of the SHS was set to 200 °C and 300 °C, with flow rates of 5 m/s and 7 m/s, and treatments were cycled either once or three times. As a result, increased temperature (200 °C–300 °C) and number of treatments (1 time to 3 times) led to a significantly larger microbial reduction on the surface of radish seeds. E. coli O157:H7, S. enterica, and L. monocytogenes were reduced by 4.42, 4.73, and 3.95 log CFU/g (P < 0.05), respectively, after three SHS treatments at 300 °C and 7 m/s. However, due to the ongoing potential for recovery of residual microorganisms, further research involving combinations is essential to enhance the microbicidal effect. Water imbibition showed significantly higher values in the SHS-treated group up to 30 min, indicating faster germination rates in the SHS-treated group (71.3–81.3%) compared to the control group (52.7%) on the second day, indicating a significant enhancement in germination rate. In addition, the salinity resistance of the radish seeds increased after SHS treatment. When moisturized with 0.5% NaCl solution, more radish seeds germinated after treatment with SHS (40%) than controls (22.7%) (P < 0.05). The results of this study, the first to apply semi-continuous SHS to seeds, are expected to serve as a cornerstone for future pilot-scale investigations aiming to implement the system within the seed industry.